The air has knows no boundaries. If there are elevated concentrations of a particular pollutant in a particular location, it does not mean that the source of that pollution has to be somewhere nearby. When pollutants are emitted from a particular source, they spread (disperse) into the surrounding area, and at the same time they may react with each other in different ways in the air, giving rise to different types of pollutants. Long-range transport refers to the dispersion of substances through the air to the surroundings over a longer distance, with a threshold of > 100 km being the most commonly used.
The dispersion of substances in the air depends on a number of factors, including:
Assessing the nature and significance of long-range transport of pollutants is a very complex issue. Specific models are used which take into account a number of factors, theoretical knowledge of regional and local air flow, knowledge of meteorological and dispersion conditions, the nature and location of air pollution sources in specific locations, knowledge of air pollutant reactions, etc.